1. Field of the Invention
This invention relates generally to use of the Global Positioning System ("GPS") for providing navigational information to afloat, airborne, or ground based vehicles. The invention more particularly relates to a Global Positioning System having an Attitude/Azimuth Determining System for providing a vehicle with accurate position, velocity, time, attitude, heading, pitch, and roll information.
2. Description of the Background Art
The Global Positioning System ("GPS") was conceived, developed, and deployed by the United States Department of Defense over ten years ago. The GPS' purpose was to provide air, ground, and naval users with precise position and time information. Although principally conceived for military purposes, the GPS also includes a band of signals designated for non-military use. These non-military signals can be used advantageously in numerous private and commercial applications. For example, the GPS can be used by commercial airlines and private pilots to accurately and quickly ascertain their location. Similarly, both commercial shippers and private boatmen can use the GPS at any time to accurately determine their position without the need for conventional points of reference such as the magnetic poles.
The GPS is a series of 24 satellites orbiting the earth. The satellites are deployed in twelve hour orbits, and are dispersed in six orbital planes. These satellites continuously emit electronic signals--telemetry--which are received by ground based or other terrestrial receivers. By simply receiving the signal from four or more orbiting GPS satellites, a properly configured receiver can accurately determine its position. This system has tremendous benefits over other positioning systems in that it relies upon no visual, magnetic, or other point of reference. These advantages are particularly important in applications such as aviation and naval navigation which traverse polar regions where conventional magnetic navigational means are rendered less effective by local magnetic conditions.
The GPS system has many advantages over standard positioning systems which it replaces. The GPS is available 24 hours a day on a worldwide basis. There is no signal degradation, or loss of quality of positioning information at higher latitudes. Magnetic deviations and anomalies common in standard positioning systems do not exist. Typical GPS systems are fabricated of standard, solid state electronic hardware, resulting in a low cost, low maintenance system having few or no moving parts and requiring no optics. The GPS does not require calibration, alignment, and maintenance such as conventional inertial measuring units. Further, normal GPS operation provides positioning with accuracies of about 100 meters for non-military uses. Military users can make use of encryption keys which yield position information within 10 meters.
Notwithstanding these advantages, there are still significant deficiencies in the capabilities of the GPS. For example, in naval applications, the GPS is incapable of providing information related to attitude, direction of travel, roll, and pitch. GPS can inform the navigator of a ship that the ship is within 100 meters from running aground, but does not provide the ship with its direction of travel and attitude which are required to avoid running aground.
Prior art attempts to provide for attitude information in the GPS have encountered problems. These problems include the inability to measure the whole carrier cycles between two antennae that form an interferometric baseline. The L1 GPS carrier frequency has a wavelength or cycle length of about 20 cm. Therefore, a baseline formed by two antennae separated by one meter can have 11 (+5 to -5) permutations of whole GPS L1 carrier cycles depending on the angle between the baseline and the line of sight vector to the GPS satellite. GPS receivers cannot measure these whole cycles, but only measure the difference in fractional phase between the antennae. If four satellites are required to calculate the three dimensional attitude of that baseline, there are 11.sup.4 possible combinations. Previous attempts to resolve this problem have focused on either exhaustive searches of all possible combinations or informed searches of the most likely integer counts. These methods are too cumbersome or unreliable for real-time navigation instruments.
The industry lacks an attitude determining system for use in conjunction with the GPS which permits navigation, hydrographic and land survey, buoy positioning, vehicle alignment, and pointing vector determination, among other attitude determining applications.